This depends very much on the camera and flash unit that you&#8217;re using. Early on, Canon put control for this feature on the flash unit. Later they switched to putting control for this feature on the camera body. So whether you have second-curtain sync available to you depends on a complicated set of permutations.

Many mid to high end Canon flash units, listed below, have a button or switch which lets you enable second curtain sync. It&#8217;s usually marked with a triple triangle ( >>> ) symbol or the word SYNC. For instance, on the 430EZ and 540EZ you press the + and - buttons together simultaneously to turn on second-curtain sync. When you do so a triple triangle symbol appears in the LCD. On the 300EZ and 300TL there&#8217;s a small slide switch - left is first-curtain sync and right is second-curtain.

Most midrange and professional EOS bodies from the A2(E)/5 onwards have a custom function that lets you specify whether you want first or second curtain flash. The exception is the original Elan/100, which had a custom function that can only control the internal flash and not external flash units. In the case of a camera with a custom function and an external flash unit which has a second curtain switch then you use the physical switch on the flash to control the function.

Low-end EOS cameras, such as the 1000 series or Rebel series, do not have any custom functions and so cannot control second curtain sync options directly. So to take advantage of second curtain sync on such cameras you must have an external flash which has externally-available controls to operate it.

Second-curtain sync cannot be used with any EOS camera in a PIC (icon) mode - you have to be set in P, Av, Tv or M modes. And you can&#8217;t set second-curtain sync in stroboscopic mode or FP mode, since that wouldn&#8217;t make any sense. Finally, second-curtain sync requires a dedicated Speedlite flash unit - it isn&#8217;t supported on flash units connected via a PC socket (see PC connector section below).

More importantly how would this work with TTL metering? The flash needs to metre first and then expose significantly later. I'm sure it all sounds easy if you have the datasheet infront of you. Would suck if you didn't.

Maybe canon also simply withhold this information from competitors to increase sales of it's own speedlights.

More importantly how would this work with TTL metering? The flash needs to metre first and then expose significantly later. I'm sure it all sounds easy if you have the datasheet infront of you. Would suck if you didn't.

Click to expand...

You can still use TTL metering and 2nd curtain sync (with a Canon flash). The pre flash fires before the exposure as usual...but then the actual flash doesn't fire until the end of the exposure....so the longer your exposure, the longer the delay. This can be troublesome when shooting people because they might move or blink after the first flash. Of course, if the scene changed between the metering and the flash...the exposure may not be accurate.

Maybe canon also simply withhold this information from competitors to increase sales of it's own speedlights.